Facile Proton Transport in Ammonium Borosulfate—An Unhumidified Solid Acid Polyelectrolyte for Intermediate Temperatures
M.D. Ward, Brian L. Chaloux, M. D. Johannes, Albert Epshteyn
Abstract
Abstract High proton conductivity is reported for unhumidified ammonium borosulfate, NH 4 [B(SO 4 ) 2 ], a solid acid coordination polymer that contains 1D, hydrogen‐bonded NH 4 + ··· 1 ∞ [B(SO 4 ) 4/2 ] − chains. NH 4 [B(SO 4 ) 2 ] is thermally stable to 320 °C and is amenable to sintering into monolithic, polycrystalline discs at 200 °C and about 300 MPa of uniaxial pressure. Impedance spectroscopy measurements reveal ionic conductivities for sintered ammonium borosulfate of 0.1 mS cm −1 at 25 °C and up to 10 mS cm −1 at 180 °C in ambient air. No superprotonic transition is observed in the temperature range of 25–180 °C. Ab initio molecular dynamics simulations show these high conductivities are aided by free rotation of the NH 4 + units and significant gyrational mobility of the SO 4 tetrahedra, which, in turn, provide facile pathways for proton locomotion. High conductivities, a wide operational temperature window, and tolerance to both ambient and anhydrous conditions make NH 4 [B(SO 4 )] 2 an attractive candidate electrolyte for intermediate‐temperature hydrogen fuel cells that may enable operation at temperatures as high as 300 °C without active humidification.